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Alcock-coupland_mst2006 - INSTITUTE OF PHYSICS PUBLISHING...

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I NSTITUTE OF P HYSICS P UBLISHING M EASUREMENT S CIENCE AND T ECHNOLOGY Meas. Sci. Technol. 17 (2006) 2861–2868 doi:10.1088/0957-0233/17/11/001 A compact, high numerical aperture imaging Fourier transform spectrometer and its application R D Alcock and J M Coupland Wolfson School of Mechanical and Manufacturing Engineering, Loughborough University, Loughborough, LE11 3TU, UK Received 27 April 2006, in final form 24 July 2006 Published 28 September 2006 Online at stacks.iop.org/MST/17/2861 Abstract This paper describes a compact imaging Fourier transform spectrometer with high numerical aperture. In comparison with other optical arrangements in which extended interferometer paths are required for the inclusion of dispersion compensation optics, this technique utilizes a rudimentary cubic beam splitter based Michelson interferometer with minimal optical path so that the numerical aperture of the system is maximized. Mathematical modelling is presented showing that the fringe distortions caused by the dispersion in the cubic beam splitter can be entirely removed without any loss of the spectral information. An illustration of the power of the technique is given classifying between different plant foliage performed using a Fisher discriminant function based optimal linear filtering. Keywords: imaging Fourier transform spectrometry, Michelson interferometer, spectroscopy 1. Introduction Spectroscopic, multi-spectral or hyper-spectral imaging has attracted considerable scientific interest in recent years in applications as diverse as fluorescent microscopy, astronomy, remote sensing of the earth and ballistic analysis. The basis of spectroscopic imaging is that objects may be classified according to not only their shape but also spatially varying spectral characteristics. It is often the case that these characteristics contain significantly more detail than that described by just the traditional red–green–blue ‘tri-stimulus’ approach [ 1 , 2 ]. Furthermore, useful information is not limited solely to the visible spectrum, but extends into the so-called chemical fingerprint region of the infrared and ultraviolet parts of the electromagnetic spectrum. Due to their inherent simplicity, filter based hyper-spectral imaging techniques such as acousto-optic tuneable filters remain popular [ 3 , 4 ]. As their name suggests, however, they are light inefficient discarding all but the wavelength of interest. Dispersive CCD arrays [ 5 , 6 ] on the other hand are more light efficient, however require the scene of interest to be scanned as one spatial dimension on the CCD is sacrificed to record the spectrum. Consequently, state-of-the-art imaging systems have been proposed that build upon Fourier transform spectroscopy interference techniques whereby spectral information is cosine encoded as a function of interferometer path length difference. The fundamental strength of Fourier transform spectrometry is that approximately 50% of the light progressing through the optical arrangement is collected in the recorded image.
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